In a Dec. 23, 2009 photo, Delaware Attorney General Beau Biden holds a press conference at the Department of Justice in Georgetown, Del. to update the charges against Dr. Earl B. Bradley. Delaware Attorney General Beau Biden will not be running for the U.S. Senate seat once held by his father, Vice President Joe Biden. (AP Photo/Chuck Snyder)

Dover, Del. (AP) - Beau Biden announced Monday that he will not seek election to the U.S. Senate seat long held by his father, Vice President Joe Biden, putting another Democratic-held Senate seat in jeopardy and dealing another blow to President Barack Obama's flailing party.

The Delaware attorney general told supporters in an e-mail that he will run for re-election to his state post instead of running against GOP Rep. Mike Castle for the seat the elder Biden held for 36 years. He cited a need to focus on prosecution of high-profile child molestation case.

"I have a duty to fulfill as attorney general, and the immediate need to focus on a case of great consequence. And that is what I must do," Biden, 40, wrote. "Therefore I cannot and will not run for the United States Senate in 2010."

He left open the door of a candidacy in future years.

Biden's decision makes the Democratic-held seat vulnerable as the Democratic Party and its leader, Obama, are licking their wounds following Republican Scott Brown's victory last week for the Senate seat in Massachusetts once held by Democratic Sen. Edward M. Kennedy; the GOP upset ended the Democrats' filibuster-proof majority in the Senate.

Two weeks before that, Sen. Byron Dorgan chose to retire in North Dakota rather than face re-election, putting a once-safe Democratic seat in serious trouble. No less than three Democratic senators were already vulnerable, including Senate Majority Leader Harry Reid of Nevada, Blanche Lincoln of Arkansas and Michael Bennet of Colorado.

The GOP also is making a play for the Illinois seat once held by Obama; Sen. Roland Burris, who was appointed to the seat, is not running. And Republicans are planning to fight for the Senate seat held by the retiring Christopher Dodd in Connecticut. Given a political environment tipping their way, the GOP also is keeping an eye on Sens Barbara Boxer in California, Arlen Specter in Pennsylvania and Kirsten Gillbrand in New York.

Biden's decision was a surprise, given that his father's confidant and former Senate chief of staff, Ted Kaufman, was appointed to the seat by Delaware's governor essentially to keep it warm for the son until he was able to run.

But then Castle, a former two-term governor and one of the most successful politicians in Delaware history, entered the race, dramatically increasing the likelihood of a competitive race.

With Biden declining a run, Democrats in Washington said they were turning to New Castle County Chief Executive Chris Coons in hopes he would run. Coons was noncommittal last week when asked whether he would seek the nomination if Biden bows out, saying only that he looked forward to supporting Biden.

While Obama didn't call the younger Biden, White House officials said the president and vice president discussed the race regularly, as recently as Friday. Obama had asked his No. 2 to tell Biden the president's belief that he would win if he ran. They spoke on condition of anonymity to discuss the private conversations.

Since returning home in September after a yearlong deployment to Iraq with his National Guard unit, Beau Biden had been focused on his family and his job as attorney general.

In recent weeks, his agency has been enmeshed in the case of a Delaware pediatrician charged with sexually assaulting several of his patients. Prosecutors believe Dr. Earl Bradley of Lewes, who was arrested in December, may have molested more than 100 children over the past decade.

"The reality is, it became increasingly clear over the last several weeks that it was impossible to mount a Senate campaign in the face of dealing with both the prosecution in Lewes as well as the things I need to do, our office needs to do, for victims," Biden told The Associated Press.

Biden vowed while campaigning for attorney general in 2006 that he would crack down on child predators, and he made the creation of a separate child predator unit within the state Justice Department his top priority after winning office. Had he turned his attention from the Bradley prosecution to running for Senate, he likely would have faced criticism from some voters, but Biden told the AP he was not thinking in such terms.

"I knew I had to be entirely focused on this case and the victims in it," he said. "I sought this office and ran for this office to create a child predator unit. This is one of the reasons why I wanted to be attorney general."

Biden said he and his wife, Hallie, made the decision before informing family members, who he said were supportive of their decision.

Castle, 70, announced in October that he would not seek a 10th term in the House but would run for this fall's special election to fill the remaining four years of the Senate term Joe Biden won in 2008.

Castle, a leader of GOP centrists who has demonstrated crossover appeal among Democrats as well as unaffiliated voters in Delaware, has a significant head start over the Democrats in fundraising. He has taken in more than $1 million since announcing his Senate bid in October, ending 2009 with about $1.7 million in his campaign chest.

Biden ended 2009 with slightly less than $100,000 in his attorney general campaign fund, having raised only about $6,500 in cash contributions during the year.

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AP National Political Writer Liz Sidoti in Washington contributed to this report.